A display for displaying images is one of light-emitting devices necessary in modern life, which takes various configurations, such as so-called TV monitors, liquid crystal displays that have been developed rapidly in recent years, and organic EL displays that are expected to develop future, to meet requirements. Above all, an organic EL (Electro Luminescence) display has been most attracting attention as a next-generation flat panel display device.
In the light-emission mechanism of a light-emitting element composing an organic EL display, by locating a light-emitting layer comprising a light emitter composition of between electrodes and applying a current, an electron injected from a cathode and a hole injected from an anode are recombined in a luminescent center of the light-emitting layer to form a molecular exciton, and a photon emitted when the molecular exciton returns to the ground state is used. In principle, the energy difference between the excited state and the ground state (hereinafter, referred to as a band gap) corresponds to a maximum wavelength of luminescence. Therefore, the band gap most contributes to an luminescence color.
In order to utilize a light-emitting element as a full-color display for displaying images, regions capable of emitting at least light's three primary colors of red, blue, and green (hereinafter, referred to as “pixels”) may be provided independently, and the individual pixels may be made to emit light at the right and appropriate time. In this case, the luminescence colors of light emitter compositions to be used for the individual pixels come down to affect the quality of the display most greatly. Therefore, a luminescent material capable of emitting light of a red, blue, or green color with favorable color purity is required. The favorable color purity mentioned here is evaluated by x and y coordinate values in CIE chromaticity coordinates. As the blue color, positive x and y values near 0 are ideal. As the green, x about 0.5 and a y value about 0.8 are ideal. At the same time, as the red, x about 0.7 and y about 0.25 are ideal. These ideal values are not always needed, or are not actually achieved. However, these values are ideal in order to recreate more natural colors.
Further, the individual light emitters are required to have various performances, specifically such as electrochemical stability, chemical stability, thermal stability, a low degree of crystallinity, and a high carrier transporting property. Unless these requirements are met, it is not possible to manufacture a light-emitting element with a long life even if luminescence with favorable color purity can be obtained.